1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a method of fabricating a liquid crystal display having a storage capacitor with increased capacitance.
2. Description of the Related Art
Generally, liquid crystal displays (LCD) display a picture by controlling light transmittance using electric fields. To this end, an LCD includes a liquid crystal display panel having liquid crystal cells arranged in a matrix, and a drive circuit to drive the liquid crystal display panel. The liquid crystal display panel is provided with a common electrode and pixel electrodes to apply an electric field to each liquid crystal cell. Typically, pixel electrodes are formed on a lower substrate in liquid crystal cells, whereas the common electrode is integrally formed on the entire surface of an upper substrate. Each pixel electrode is connected to a thin film transistor (TFT) that is used as a switching device. The pixel electrode along with the common electrode drives the liquid crystal cell in accordance with a data signal applied via the TFT.
Referring to FIGS. 1 and 2, a liquid crystal display includes a TFT TP arranged at each crossing of data lines 4 and gate lines 2, a pixel electrode 22 connected to a drain electrode 10 of the TFT, and a storage capacitor SP located at an overlapping part of the pixel electrode 22 and the gate line 2.
The TFT TP includes a gate electrode 6 connected to the gate line 2, a source electrode 8 connected to the data line 4, and a drain electrode 10 connected to the pixel electrode 22 through a drain contact hole 20. Further, the TFT TP includes semiconductor layers 14 and 16 to form a channel between the source electrode 8 and the drain electrode 10 by a gate voltage applied to the gate electrode 6. Such a TFT TP responds to a gate signal from the gate line 2 to selectively apply a data signal from the data line 4 to the pixel electrode 22.
The pixel electrode 22 is located at a cell area partitioned off by the data line 4 and the gate line 2, and is made from a transparent conductive material having a high light transmittance. The pixel electrode 22 generates a potential difference together with a common electrode (not shown) formed at an upper substrate (not shown) by the data signal applied through the drain contact hole 20. This potential difference causes liquid crystals located between the lower substrate 1 and the upper substrate (not shown) to rotate by their dielectric anisotropy. Accordingly, a light applied from a light source via the pixel electrode 22 is transmitted toward the upper substrate.
The storage capacitor SP limits voltage variation in the pixel electrode 22. The storage capacitor SP includes a gate line 2, and a storage electrode 24 formed to overlap the gate line 2 with a gate insulating film 12 therebetween. The storage electrode 24 is electrically connected to the pixel electrode 22 through a storage contact hole 26 formed on a protective film 18.
A method of fabricating the liquid crystal display will be described in conjunction with FIGS. 3A through FIG. 3E.
First, a gate metal layer is deposited on the lower substrate 1 and then patterned to form the gate line 2 and the gate electrode 6 as shown in FIG. 3A. A first insulating material is deposited on the lower substrate 1 provided with the gate line 2 and the gate electrode 6, thereby forming the gate insulating film 12 as shown in FIG. 3B. First and second semiconductor materials are deposited on the gate insulating film 12 and then patterned to form an active layer 14 and an ohmic contact layer 16.
Subsequently, a data metal layer is deposited on the gate insulating film 12 and then patterned to form the data line 4, the storage electrode 24, the source electrode 8 and the drain electrode 10 as shown in FIG. 3C. Thereafter, a second insulating material is deposited thereon to form a protective film 18 as shown in FIG. 3D, and then the drain contact hole 20 and the storage contact hole 26 are formed to pierce the protective film 18.
A transparent conductive material is deposited on the lower substrate 1 provided with the protective film 18 and then patterned to form the pixel electrode 22 as shown in FIG. 3E.
The storage capacitor SP of the related art liquid crystal display is made up of the storage capacitor 24 and the gate line 2, which overlap each other having the gate insulating film 12 therebetween. In this case, the process becomes more complicated since a separate storage electrode 24 needs to be formed for the storage capacitor, increasing the probability that a pattern defect may be generated.
In order to solve such a problem, the storage capacitor shown in FIGS. 4A and 4B is made up of the pixel electrode 22 and the gate line 2, which face each other with the gate insulating film 12 and the protective film 18 therebetween. In this case, since it is not necessary to form a separate storage electrode 24, there is an advantage in that the probability of generating the pattern defect is reduced.
However, the distance between the electrodes 2 and 22 is greater when the storage capacitor is made up of the pixel electrode 22 and the gate line 2, with both of the gate insulating film 12 and the protective film 18 therebetween, than when the storage capacitor is made up of the gate line 2 and the storage electrode 24 with only the gate insulating film 12 therebetween. Accordingly, there is a problem in the related art having both of the gate insulating film 12 and the protective film 18, in that the capacitance of the storage capacitor is reduced because capacitance is inversely proportional to the distance between the electrodes 2 and 22. In particular, when the protective film 18 is formed of an organic insulating material, as shown in FIG. 4B, to increase aperture ratio, the protective film 18 is relatively lower in dielectric constant and greater in thickness than when the protective film 18 is formed of an inorganic insulating material as shown in FIG. 4A. Therefore, there is a problem in that the capacitance of the storage capacitor is reduced more because capacitance is proportional to dielectric constant and inversely proportional to the distance between electrodes.